The cognitive wireless to recognize a surrounding wireless environment and to optimize a communication parameter on the basis of the wireless environment is well known. As an example of the cognitive wireless, a case that a secondary system (interference causing system) shares a frequency band which is assigned originally to a primary system (interference receiving system) is exemplified.
For example, it is discussed in IEEE 802.22 Draft Ver 2.0 to standardize a WRAN system which is corresponding to the secondary system and which shares a frequency band (TV channel) assigned originally to a TV broadcasting system corresponding to the primary system. Here, IEEE is an abbreviation of Institute of Electrical and Electronic Engineers. Moreover, WRAN is an abbreviation of Wireless Regional Area Network.
When the secondary system shares the frequency band with the primary system, it is necessary that the secondary system does not cause any influence on an existing service which the primary system provides. In order to avoid interference to the primary system, the secondary system must use a frequency band which the primary system does not use temporally and spatially or the secondary system must communicate with making an amount of the interference not larger than an amount of the interference permissible in the primary system.
For example, a service area of the TV broadcasting system, which is corresponding to the primary system, is defined as an area whose receiving radio wave intensity is not smaller than a predetermined level (for example, not smaller than 60 dBμV/m). Then, it is necessary that the secondary system (for example, the above-mentioned WRAN system) suppresses the interference so that a receiver, which exists in the service area, may maintain predetermined SIR (for example, 23 dB). Here, SIR is an abbreviation of Signal to Interference power Ratio.
As an example of an art to suppress the interference which the secondary system causes to the primary system, an art disclosed in a non-patent document 1 is exemplified. The non-patent document 1 describes a method to identify the service area of the TV system by use of a TV database, and to determine transmit power by use of propagation loss, which is estimated on the basis of a propagation model, so that interference power at an edge of the service area may be permissible.
Moreover, a non-patent document 2 describes a wireless communication system shown in the following. According to the wireless communication system, the primary system detects the interference through measuring a decline of throughput, and issues an interference alert to the secondary system in the case of detecting the interference. Then, the secondary system stops using the frequency which the secondary system shares with the primary system.
Moreover, a patent document 1 describes an art shown in the following. According to the art, one wireless apparatus estimates propagation characteristic of a signal (estimate an attenuation coefficient due to a distance), which is transmitted by a transmitter of another system, by use of received interference power (corresponding to received power of the primary signal) from the other system. Then, the wireless apparatus identifies an edge of an area whose received power is not smaller than a desired value, and makes its own transmit power increasing so as to become very close to a permissible value at the edge of the area.